I'm going to disagree with you on one point in particular. Performance is always proportional to practice. You will not be 'more skilled' at a job that you spend less time playing. In real life, this concept has repeatedly been proven in fields which are procedurally focused. High case volumes invariably translate into better outcomes. It's also why people who work in such fields (like pilots and surgeons) worry a lot about deskilling. You don't really benefit from being a generalist. What people traditionally attribute to 'talent' is just a reflection of the fact that being repeatedly exposed to a skill earlier in life accelerates your skill acquisition. But it's all just hard work in the end, and you only get better with repeated exposure to a task and only get worse with prolonged time spent away.

In this game, you may find that certain skills translate across jobs, like maintaining uptime as melee. But you're not going to become the world's best dragoon player by playing it less than your competition. And there's always an opportunity cost of time you spend on other jobs. I'm not saying that you can't play multiple jobs well, but you're not going to understand all of them at the same level, and certainly not at a level that's going to let you comment fairly on all their respective depths.

I don't like commenting on 'job difficulty' because it's disrespectful to the work put in by players who spend most of their time on that job. Some people have been playing this game for over 10 years with their primary focus on a single job. Who am I to comment on the difficulty level of 'MNK' across the years when it's not a job that I've ever 'mained' in prog (outside of a brief but enjoyable stint as part of a T8 solo tank strat)? I can play it competently in most standard content due to translatable skills from jobs I have mained, but I won't pretend to be an expert. When I see a particularly vocal section of this forum go on about how much better they are than everyone else I feel tempted to respond, but other than that I think it's better to take the jobs as just a canvas that you can display skill at a variety of levels. It's just mutual respect. You can play a given job at a low level or a very high level, regardless of what your preference is. There are no 'high skill jobs', only 'skilled players'. There is always little tricks that you've never thought about. One of the most skilled tank demonstrations that I've ever seen was actually on Heavensward PLD of all things, in watching someone reposition Quickthinx while constantly maintaining directional autos and swivel locking around to Shield Bash a heart. It's easy to dismiss something that you lack sufficient skill in as being 'easy' simply out of ignorance.

That's why I believe that there shouldn't be official validation of any of this. It's possible that, if you had the ideal player who could play all jobs mathematically perfectly, they would find some jobs easier to achieve that with than others. But that player doesn't exist. So rather than officially sanctioning a particular job as being higher rDPS because it is 'officially more difficult', just level the playing field and refuse to officially take a stance on 'difficulty'. The focus should be on designing actions and fights that allow for more variety in skill expression. Interesting movement abilities. Fights with more movement and positioning.